Generally, coated conductors are tape-like conductors having a multilayer composition with a substrate, a superconductor layer and a varying number of buffer layers between the substrate and the superconductor layer. The buffer layers serve to compensate for the various different properties of the material used. Typical coated conductor structures require several buffer layers.
Though not restricted thereto nowadays the rare earth barium cuprate type-superconductors of the formula REBa2Cu3O7-x are conventionally used in the production of coated conductors. A particular member thereof is that one known by the reference YBCO-123 wherein the numerical combination 123 stands for the stoichiometric ratio of the elements Y, Ba and Cu.
One major problem in the production of such high-temperature superconductors is the orientation or alignment of the crystal grains of the superconductor material which should be as high as possible in order to have high current carrying properties such as critical current density (Jc) and critical current (Ic) in the super-conducting state. Preferably the superconductor layer has a biaxial texture with the crystal grains being aligned both parallel to the plane of the layer (a-b alignment) and perpendicular to the plane of the layer (c-axis alignment).
The quality of the biaxial texture is typically expressed in terms of the crystallographic in-plane and out-of-plane grain-to-grain misorientation angle. For providing good superconductor performance said angle should be as small as possible. Preferably the angle should not exceed 9° for obtaining superconductor properties sufficient for practical use.
The degree of texture, i.e. the sharpness of the texture, can be determined by X-Ray diffraction specifying the in-plane and out-of-plane orientation distribution function of the grains of the layer.
Based on the X-ray data the values of the full-width-half-maximum (FWHM) of the in-plane phi scan (Δφ) and out-of-plane rocking curve (Δω) can be obtained. The smaller the respective FWHM-values the sharper the texture.
Orientation of a layer to be grown can be achieved by epitaxial growth. Epitaxial growth refers to a process wherein the layer to be grown adopts the crystallographic orientation of the substrate or layer onto which the layer is formed.
That is, the crystallographic orientation of the layer grown is directly related to the crystallographic orientation of the underlying layer onto which said layer is deposited.
Consequently, for epitaxial growth a suitably oriented surface must be provided which can serve as template for the desired orientation to the superconductor layer deposited thereon.
According to the simplest approach a single crystal substrate could be used for achieving the desired biaxial texture of the superconductor layer. However, single crystal substrates are generally expensive and have only poor mechanical properties. Further, the surface area is only limited so that large scale production of coated conductors with long lengths as required for practical applications is not possible.
According to the so-called IBAD (ion beam assisted deposition) approach, a highly textured buffer layer is deposited by ion beam assisted deposition on a randomly oriented metallic substrate, said buffer layer serves to transfer the desired texture to the superconductor layer grown onto the buffer layer. IBAD relates to a vacuum deposition technique requiring specific equipment. Further, by such vacuum techniques coating of substrates of long lengths is difficult.
According to yet another approach substrates are used which have been biaxially textured, for example, by mechanical deformation followed by recrystallization annealing. A specific example for such a method is called RABiTs (rolling assisted biaxially textured substrates). On such textured substrates as obtained by RABiTs buffer layers with suitable texture can be deposited, which, in turn, can serve as template for transferring the desired texture to a superconductor layer to be grown on the buffer layer. Examples of metals suitable as substrate are copper, nickel, silver, iron and alloys thereof etc.
However, this approach using biaxially textured substrates has the drawback that intermediate buffer layers are required between the buffer layer serving as template (in the following referred to “template buffer layer” or “template”) and the superconductor layer for example for preventing chemical reaction between the substrate material and material of the superconductor layer, for avoiding oxidation of the metal surface of the substrate which is particularly a problem in case of metal substrates, etc. Usually several different buffer layers are required between the template buffer layer and the superconductor layer.
Typical buffer layers are oxides and include cerium oxide, yttrium-stabilized zirconia (YSZ), strontium titanium oxide, rare-earth aluminates and various rare-earth oxides.
For example, a typical RABiTs based YBCO coated conductor has an architecture of NiW/Y2O3/YSZ/CeO2/YBCO.
Methods for growing buffer layers for coated conductors are well known to those in the art and include for example, vacuum methods, such as physical vapour deposition (PVD), for example pulsed laser deposition (PLD), electron beam evaporation and sputtering as well as non-vacuum deposition processes such as chemical solution deposition, chemical vapour deposition (CVD) and metal organic chemical vapour deposition (MOCVD).
Apart from the need of several buffer layers the buffer layer serving as template must be of high quality with a close lattice match to the superconductor material to be grown in order ensure the formation of the desired high crystalline orientation of the superconductor layer.
“Close lattice match” requires similar crystal structure of the template and the superconductor layer to be grown thereon with the lattice parameters of the template being close to the lattice parameters of the superconductor layer to be grown thereon in order to allow the desired epitaxial growth of the superconductor layer.
Only if there is sufficient lattice match, transfer of the texture of the template to the superconductor layer to be grown thereon may occur.
However, there is a particular problem, that usually the sharpness of the texture of the obtained layer, here the superconductor layer, is reduced compared to that of the underlying template layer, i.e. the respective FWHM-values are larger. Consequently, for obtaining a sufficiently sharp texture of the superconductor layer template layers are required with a texture as sharp as possible in order to allow compensation of the texture loss in the superconductor layer to be grown thereon.
There are some attempts in the art to reduce the number of buffer layers and to improve lattice match of the buffer layer with the superconductor layer in order to support the transfer of texture. However in none of these documents the loss of texture sharpness of the superconductor layer grown compared to the underlying template layer is addressed.
For example, EP 1 178 129 is related to a YBCO coated conductor obtained by IBAD using a non-oriented substrate wherein the buffer layer serving as template for transferring the desired orientation to the YBCO layer is deposited by ion beam assisted deposition.
It is disclosed that the number of buffer layers in a coated conductor composed of a non-oriented substrate, a first buffer layer made of YSZ, a second buffer layer made of Y2O3 and a YBCO-superconductor layer can be reduced by replacing the first and the second buffer layer by a buffer layer made of a compound having the general formula RE2Zr2O7 or RE2Hf2O7 with RE being a rare earth element selected from lanthanum and the lanthanoids. As discussed here the problem with a buffer layer made of YSZ is that during heat treatment, necessary in the formation of the final superconductor layer, diffusion of components of the superconductor layer and the YSZ layer is likely to occur. Such diffusion is prevented by the provision of the Y2O3 layer.
Due to smaller reactivity of RE2Zr2O7 or RE2Hf2O7 compared to YSZ provision of an additional layer for avoiding diffusion is no longer necessary.
As a consequence, the number of buffer layers can be reduced. Further, it is disclosed that by suitable selection of the component Re the lattice constants of the resulting buffer layer can be adjusted to those of the superconductor layer to be grown in order to obtain buffer layers having a close lattice match to the superconductor layer.
There is a discussion of the deposition parameters of the buffer layer in order to obtain a reduction of the missorientation angle of said buffer layer.
According to EP 1 178 129 fairly thick buffer layers of 1000 nm are required.
As in EP 1 178 129 U.S. Pat. No. 6,399,154 discloses to replace the conventionally used YSZ buffer layer for transferring orientation to the YBCO layer to be deposited by RE2Zr2O7 (REZO) in view of the possibility for adjusting the lattice parameters of the obtained buffer layer as close as possible to the lattice parameters of the YBCO layer as well as of the possibility for reducing the number of layers. Contrary to EP 1 178 129 U.S. Pat. No. 6,399,154 relates to the RABiTs approach using a textured metal substrate, and the template buffer layer is formed by a metal organic deposition technique, such as the sol-gel route.
Though reference is made to direct growth of a YBCO film onto the buffer layer there is no disclosure of the suitability of such directly grown YBCO-layer in coated conductors. Moreover, in the examples in addition to the ReZO layer further buffer layers are used such as YSZ and CeO2. This is consistent with the results of T. G. Chirayil et al, “Epitaxial growth of La2Zr2O7 thin films on rolled Ni-substrates by sol-gel process for high Tc superconducting tapes” in Physica C336 (2000) 63-69. As stated herein attempts to deposit YBCO (300 nm) directly on a sol-gel processed LZO buffer layer of 60 nm thickness results in YBCO films with decreased critical temperature. The reason therefore is seen in the presence of nickel oxide suggesting Ni diffusion from the substrate into the buffer layer.
S. Sathyamurthy et al., Lanthanum zirconate: A buffer layer processed by solution deposition for coated conductor fabrication, in J. Mater. Res., Vol. 17, No. 9, September 2002, pages 2181 to 2184 relates to the replacement of the conventionally used multilayer buffer architecture by only a single buffer layer which can be deposited using a scalable technique.
Reported are the results of using a single lanthanum zirconate La2Zr2O7 buffer layer deposited by sol-gel processing for YBCO coated conductor, the YBCO layer being deposited by pulsed laser deposition (PLD).
As substrates textured Ni and Ni—W tapes, respectively, are used.
WO 2006/015819 A1 relates to a “all solution process” wherein the buffer layers as well as the superconducting layer are deposited by means of chemical solution deposition. It was the object to avoid conventionally used physical processes such as pulsed laser deposition (PLD) for depositing superconducting layers.
WO 2006/015819 A1 gives no explanation as to any particular deposition method for the YBCO layer. However, as far as reference to the deposition of the YBCO layer is made it is pointed to the “all solution process”, which is a clear indication, that also the YBCO layer is obtained by chemical solution deposition.
In J. L. Mac Manus-Driscoll et al., YBa2Cu3O7 Coated Conductor Grown by Hybrid Liquid Phase Epitaxy, in IEEE Transactions on Applied Superconductivity, Vol. 17, No. 2, June 2007, pages 2537 to 2541 and A. Kursumovic et al., Hybrid liquid phase epitaxy processes for YBa2Cu3O7 film growth, in Superconductor Science and Technology, 17 (2004), pages 1215 to 1223 coated conductors are discussed using a single crystal substrate on one side and a biaxially textured Ni-substrate made by RABiTS on the other side.
The HLPE-process discussed requires the provision of a liquid BaO—CuO flux layer.
With respect to the results obtained for the Ni substrate made by RABiTS technique it is stated, that growth in the optimal regime was hampered by excessive flux attack. To this, it is stated that “substrate/buffer dissolution can occur . . . ”.
Buffer dissolution, however, would affect the texture quality and, thus, the suitability of the buffer layer for transferring biaxial texture to the YBCO layer to be grown.